Flake Soil is a breeding substrate which has been fermented and composted and is suitable für beetle larva and other intervebrate animals like woodlouses and millipedes. It also serves very good as a substrate for egg deposition and as a place of retreat for adult animals.
Who mixes Flake Soil with other substrate (not recommended) or additives is acting at his own risk.
Before shipping Flake Soil is dried down a little bit due to weight reduction and storing stability. So before using it it needs to be tailored to the right humidity you need.
An adequate aeration by using Flake Soil in the breeding box is essential and very important as microorganisms and beetles/larvae both have to breeth oxygen.
A detailed description you can also find at What is Flake Soil?